The only way I am able to bring the Wan back up is to change the WANs Mac address then power cycle the Modem. So far it has happened 2 days in a row now at almost the exact same time. The modem (Motorola sb6121) and router Asus RT-N66R by the way are brand new.

Very picky indeed. I use comcast and when I fully reset my modem, I get a 1 hour lease. Once that's up I get something like a 5 hour lease and another at 1 day until finally I get 3 days 20 hours 7 minutes 11 seconds. Several times I've also been placed on completely different subnets. I experience this every firmware upgrade/swap.

Thanks RMerlin but that is definately not what is happening here. This router will simply drop its WAN dhcp settings and be unable to renew them unless I do the things I have described in my previous post. If it is simply renewing the dhcp lease from my ISP the modem should not require a power reset unless the hardware address of the router has changed.

What happens when you are disconnected and reinstall the firmware? Mine magically reconnects. Its like something in the fw resets on reinstall/reboot. Nothing happens on a plain reboot. Funny I have the same cable modem sb6121.

Try leaving your modem off for 5-10 minutes, to ensure it gets fully reset at your ISP's end. Some cable modem ISPs can be picky with allocating WAN leases.

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I was having similar issues with disconnects from my SB6121 cable modem a few months ago and called support. I hadn't had any problems for over a year and then this year in early August 2012 it seemed I began getting disconnects and the WAN IP wouldn't update. I can't say whether these disconnects had anything at all to do with the Asus router or firmware updates because I hadn't had problems with the RT-N66U router and the cable internet for the month or two prior to these incidents. The cable ISP support told me that if unplugging the power from the modem for 5-10 minutes didn't work that I should also try physically unscrewing the cable from the SB6121 cable modem as well as unplugging power for 5-10 minutes.

The support person didn't have a logical explanation as to why also physically disconnecting the cable worked for so many cable customers who couldn't renew their connection but I would theorize that the cable company equipment may sense the physical connection even when the cable modem power is off and sometimes they may not renew the WAN IP or maybe the modem holds onto the old WAN IP and won't renew even when the power is off. This is the only logical theory I can come up with why physically removing the cable would work.

I have done this twice to successfully correct the problem of loss of connection and unable to renew when only powering the modem off for 5-10 min didn't seem to work. One of those times cable company support told me that they had been working on the network in my area when my internet was down for an hour. The other time I didn't call support and I just did the same dual procedure of power off and physically disconnect the cable from the modem for 5-10 min.. Then when I reconnected the cable and plugged the power back into my SB6121 it was connecting up and working perfectly. That was about a month ago and I haven't had any problems since then. I will have to wait and see if this problem happens again in the future.

Another note of possible importance is that since July 2012 many U.S. cable companies have updated their ISP terms of agreement and they are now more closely monitoring and recording customer internet access...sites accessed by customers, equipment, etc.. There may be underlying U.S. internet laws requiring larger ISPs to monitor their customers access and usage. Many larger ISPs are moving away from allowing customer owned modems and equipment and requiring the use of their modems. Anyone who is using the newer SB6141, SB6121 or other personally owned cable modem should make sure that their cable company has properly provisioned it and included their latest updates for that equipment.

This problem occurs when I register DDNS asuscomm.com. When I turn on the cable modem CISCO and router RT-AC66U (firmware 3.0.0.4.266.23b) the internet is not.
Then I have to manually restart the router and everything is OK.

If I cancel the DDNS service, everything is fine after switching ON cable modem and router.

I think it's a firmware bug but I'm not an expert. Thank you for your advice. Sorry for the bad English.

I'm having some WAN disconnection issues, and first I suspected about my cable ISP service.

When I lose the connection I am able to ping my router (192.168.1.1), even my cable modem (192.168.100.1), but I am not able to ping any external server like 8.8.8.8 (Google DNS)

Initially I was solving this justing restarting my cable modem. But this is very annoying, because happens a lot of times per day.

So last time this problem occurred I reseted the log on router and pulled the WAN cable off:

When I reconnected the WAN cable:

So I tried changing to my another router (an old Apple Airport Express) and this problem never happened again.

Tried back on Asus and the issue returned.

I am trying to download the source code to look for something but Asus is very slow at this moment.

Any thoughts?

Best

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I also have this same problem with RT-N66U, RT-AC66U and Mikrotik OS Routerboard. They share exact same problem. They were unable to obtain/renew IP address from the modem (SB6121, SB6141, and uBee) once it was released. Rebooting the router doesn't work, reboot modem doesn't work either. Resetting them to defaults or changing MAC address of WAN interface didn't work either. I tried MK's forum but no one was able to find out what was the problem. From the log, all I can see is Modem (ISP) is't issue the IP. The most effective workaround for me to fix this problem is backup the settings, reflash the firmware (in recovery mode for Asus routers or system upgrade for Mikrotik system).